String motion sensors, commonly known as pickups, are installed on guitars, bass guitars, mandolins and other stringed musical instruments to convert the sound produced by the vibrating strings to an electronic signal. In various applications, the signal generated by a pickup may be modified by analog or digital processors, amplified or recorded before it is converted back to acoustic vibrations by a speaker or other output transducer. Conventional musical instrument pickups use different physical principles, including variations in magnetic reluctance, the Hall effect, and the piezoelectric effect, to detect the motion of musical instrument strings.
A large fraction of the pickups that have been manufactured to date comprise a permanent magnetic source and at least one set of ferromagnetic pole pieces that are surrounded by one or more wire coils. Pickups of this general design are commonly referred to as ‘magnetic pickups’ and are mounted close enough to the ferromagnetic strings of a guitar, bass, or other lute-type stringed instrument to induce a magnetic field in the strings. Vibrational motion of one or more strings produces corresponding magnetic flux variations in the pickup that generate an electrical output signal in the wire coil.
Analytically, a mounted magnetic pickup and the ferromagnetic strings of a musical instrument may be modeled as a magnetic circuit with a three dimensional magnetic flux distribution that changes in response to string vibrations. A mathematical analysis of magnetic pickup operation under a simplified set of operational parameters is provided in “Calculation Method of Permanent-Magnet Pickups for Electric Guitars” by G. Lemarquand and V. Lemarquand, IEEE Transaction of Magnetics, Vol. 43, pp, 3573-3578 (2007). In general, the electronic output signal of a magnetic pickup is not an exact representation of the acoustic vibrations of the strings on an instrument and the musicality of the output signal may be increased by harmonic distortion within the pickup. The harmonic spectrum of a pickup output signal is typically affected by its basic design, the winding pattern and tension of the one or more coils that surround the pole pieces, and by the ferromagnetic losses of the pickup components. The design and manufacture of a pickup with desirable musical qualities is an art in which the physical processes that affect the output spectrum are controlled and balanced.
Magnetic pickups came into common usage during the 1950's when hard ferromagnetic material and sensor technologies evolved to a point that the pickups could be economically mounted on a musical instrument. For purposes of clarity, the features of the present invention will be discussed with reference to solid body guitars with ferromagnetic strings. It will, however, be obvious to those skilled in the art that the scope of the invention is not limited to the illustrative guitar designs and magnetic pickup systems that embody features of the invention may be mounted on many different instruments. These instruments may have bodies with solid, semi-solid and hollow structures, various output frequency ranges and include, but are not limited to, bass guitars, Spanish-style guitars, 12-string guitars, mandolins, and steel guitars.
Magnetic musical instrument pickups may be classified into broad categories that reflect differences in basic design and tonal quality. Pickups in the ‘conventional single coil’ category have key design features that are shared by the devices disclosed in U.S. Pat. No. 2,612,072 issued to H. de Armond on Sep. 30, 1952, U.S. Pat. No. 2,573,254, U.S. Pat. No. 2,817,261, U.S. Pat. No. 3,236,930, and U.S. Pat. No. 4,220,069 respectively issued to Leo Fender on Oct. 30, 1951, Dec. 24, 1957, Feb. 22, 1966, and Sep. 2, 1980 and U.S. Pat. No. 2,911,871 issued to C. F. Schultz on Nov. 10, 1959. The ‘single coil’ designation derives from the fact that pickups in this category comprise a set of string-sensing ferromagnetic pole pieces with a magnetic flux that is linked by a single, string-sensing coil of wire. Some single coil pickups have pole pieces that are formed from magnetized hard ferromagnetic materials that generate the flux in the pickup. In other single coil designs a separate permanent magnet induces magnetic fields in the pole pieces. Conventional single coil pickups have no means for external noise rejection and are sensitive to external electromagnetic noise sources.
The external noise sensitivity of a magnetic pickup may be significantly reduced by adding a second wire coil to the pickup. The second coil is designed to generate an electronic output signal at its terminals with a noise component that is similar to the noise output of the first coil. Noise reduction is accomplished by connecting the two coils so that the noise signals have opposite phases.
Noise-cancelling single coil pickups have tonal characteristics similar to those of conventional single coil pickups and typically comprise a single set of string-sensing poles, a string-sensing coil, and a noise-cancelling coil that is connected to the string-sensing coil. In some designs, the noise-cancelling coil links the flux in a set of passive pole pieces. Illustrative noise-cancelling single coil pickups are disclosed in U.S. Pat. No. 7,166,793 issued to Kevin Beller on Jan. 23, 2007, U.S. Pat. No. 7,189,916 issued to Christopher I. Kinman on Mar. 13, 2007, and U.S. Pat. No. 7,227,076 issued to Willi L. Stich on Jun. 5, 2007.
Noise-reducing humbucking pickups or ‘humbuckers’ share key design features with the devices that are disclosed in U.S. Pat. No. 2,896,491 ('491) issued to Seth Lover in Jul. 28, 1959 and U.S. Pat. No. 2,892,371 ('371) issued to J. R. Butts on Jun. 30, 1959. Pickups in this category have at least two string-sensing coils that each link the magnetic flux in separate sets of string-sensing pole pieces. The magnetic field direction in the poles and the direction of signal propagation in each of the two coils are selected so that a large portion of the string-generated signals from the coils have an in-phase, additive relationship and a large percentage of the common-mode noise signals from the two coils have an out-of-phase, subtractive relationship. In many cases, the output signal amplitude of a humbucker-style pickup is greater than that obtained from single coil pickups and the output noise signal is smaller.
U.S. Pat. No. 2,976,755 ('755) issued to Clarence L. Fender on Mar. 28, 1961 describes a different type of noise reducing pickup in which the two sets of string sensing pole pieces sense the motion of different strings. As in the humbucker pickup designs of Lover and Butts, the two sets of pole pieces are magnetized in opposite directions and surrounded by separate coils that are connected in a noise-cancelling configuration. Other pickups that share the design features of the Precision Bass (P-Bass) pickups that are disclosed in the '755 patent include the Z-shaped pickups that are installed as standard equipment on the Comanche model guitars that are manufactured by G&L Guitars of Fullerton, Calif. and the split blade Stratocaster-style pickups that are manufactured by Fralin Pickups of Richmond, Va.
Each of the categories outlined above may be further subdivided according to the geometry of the string-sensing pole pieces. In the most common configuration, the string-sensing surface of the pole pieces has an approximately circular geometry and each pole piece senses the vibration of a single instrument string. In alternative configurations the vibrations of multiple strings are sensed by plates of ferromagnetic material that are at least partially surrounded by a wire coil. U.S. Pat. No. 4,364,295 issued to Willi Stich on Dec. 21, 1982, for example, discloses a humbucking design with soft ferromagnetic pole plates and the single pole piece of ‘lipstick-style’ pickups is a magnetized bar of hard ferromagnetic material.
In other designs, magnetic pickups may have multiple pole pieces that are each surrounded by a separate wire coil. These designs significantly reduce the intermodulation of signals from different strings and, in certain cases, have multiple output wires that allow the signals from each string to be separately transferred from the instrument to an external audio electronic system. The Z-coil design that is disclosed in U.S. Pat. No. 7,989,690 issued to Andrew Lawing on Aug. 2, 2011 and the noise-cancelling design that is disclosed in U.S. Pat. No. 7,427,710 issued to Koji Hara are examples of pickups that have multiple pole pieces that are surrounded by individual coils.
Electromagnetic noise from a pickup may also be reduced by connecting it to an external, noise-sensing coil in such a way that the noise signals from the external coil and the pickup are dephased. External sensing coils are typically mounted on the instrument in positions that are physically separated from the pickup and, therefore, sense an electromagnetic noise field that is different from the noise field that is detected by the pickup coils. In many cases, however, the difference between signals in the pickup and coil are small and effective noise cancellation can be achieved. Illustrative pickup systems with external noise-cancelling coils are disclosed in U.S. Pat. No. 7,259,318 issued to Ilitch S. Chiliachki on Aug. 21, 2007 and U.S. Pat. No. 4,581,974 issued to C. Leo Fender on Apr. 15, 1986.
Active circuitry is incorporated into some magnetic pickups to decrease the output impedance of the pickup, increase the output amplitude and, in some cases, modify the pickup tone. Active magnetic pickups with different coil and pole piece designs are manufactured, for example, by EMG, Inc. of Santa Rosa, Calif.
The design and manufacture of magnetic musical instrument pickups are described from an historical and lay engineering perspective in The Guitar Pickup Handbook, the Start of Your Sound by Duncan Hunter (Backbeat/Hal Leonard, New York, 2008), Pickups, Windings and Magnets and the Guitar Became Electric by Mario Milan (Centerstream, Anaheim Hills, 2007) and Electric Guitar, Sound Secrets and Technology by Helmuth Lemme (Elektor, Netherlands, 2012). On a more technical level, Engineering the Guitar, Theory and Practice by Richard Mark French (Springer, New York, 2009) contains a chapter on Guitar Electronics and a thorough treatment of musical sound quality and tone as viewed from an engineering and physics perspective. A technical analysis of the history and operation of guitar pickups is also provided by the slides from a seminar entitled “Electronic Transducers for Musical Instruments,” that was given by Dr. Steven Errede at a meeting of the Audio Engineering Society at University of Illinois at Urbana-Champaign on Nov. 29, 2005 and published on the internet at http://courses.physics.illinois.edu/phys406/Lecture_Notes/Guitar_Pickup_Talk/Electronic_Transducers_for_Musical_Instruments.pdf.